In a large bowl, combine the beef, bread crumbs, onion, eggs, salt and pepper, and mix well. Pat out meat mixture into a 14×18 inch rectangle on a piece of wax paper. Spread cheeses and spinach over the meat, leaving a 3/4 inch border around the edges. Roll up jelly roll fashion to enclose the filling and form a pinwheel loaf. Press beef in on both ends to enclose the cheese and spinach. Place in a foil lined 9×13 inch baking dish.
Bake in the preheated oven 1 hour, or until internal temperature reaches 160 degrees F.
Serve with garlic mashed potatoes, rolls, and roasted green beans.